Upper Midwest lakes are supersaturated with N2

Little is known about the exchange of gaseous nitrogen (N-2) with the atmosphere in freshwater systems. Although the exchange of N-2, driven by excess or deficiencies relative to saturation values, has little relevance to the atmospheric N-2 pool due to its large size, it does play an important role in freshwater and marine nitrogen (N) cycling. N-fixation converts N-2 to ammonia, which can be used by microbes and phytoplankton, while denitrification/anammox effectively removes it by converting oxidized, inorganic N to N-2. We examined N-2 saturation to infer net biological nitrogen processes in 34 lakes across 5 degrees latitude varying in trophic status, mixing re-gime, and bathymetry. Here, we report that nearly all lakes exam-ined in the upper Midwest (USA) were supersaturated with N-2 ( 85% of samples, n = 248), suggesting lakes are continuously re-leasing nitrogen to the atmosphere. The traditional paradigm is that freshwaters compensate for N-limitation through N-fixation, but these results indicate that lakes were constantly losing N to the atmosphere via denitrification and/or anammox, suggesting that terrestrial N inputs are needed to balance the internal N cycle.